The dbl oncogene encodes a 66-kilodalton (kDa) protein whose first 50 amino acids, derived from a different human locus, are fused to 428 amino acids encoded by the 3' end of the dbl proto-oncogene. Although both dbl and proto-dbl have transforming potential, the transforming activity of the dbl oncogene is much higher than that of the normal counterpart. A comparative study between the dbl and proto-dbl products in NIH/3T3 transfectants has been performed. The dbl oncogene is a cytoplasmic phosphoprotein localized in both the membrane and soluble fractions of the cells. On the other hand, the proto-dbl product is a 115-kDa protein. which shares with p66 the same subcellular distribution and post-translational modification. However, p115 is phosphorylated to a lesser extent than p66 and it has an extremely short half-life, while p66 is a very stable protein. The level of expression of p66 and pll5 in NIH/3T3 transfectants is low. Therefore, in order to better understand the biochemical and biophysical properties of these proteins, they have been expressed at a high level in the baculovirus system.